The present invention relates to a method of making seamless nonferrous tubing. More particularly this invention concerns the production of seamless copper tubing.
U.S. Pat. No. 4,876,870 of Rantanen describes a method of producing seamless nonferrous tubing where a tubular workpiece straight from a continuous-casting die is rolled in a planetary-roll system so that the heat created by the rolling raises the temperature of the workpiece above the recrystallization temperature. Such recrystallization forms in the workpiece a new strain-free grain structure that replaces the grain structure normally created by cold working. This system makes tubing of copper, nickel, zirconium, titanium, or alloys thereof and normally relies on a one-step thickness reduction of at least 70%. With copper a crystal size of 0.005 mm to 0.050 mm is produced when mechanical working reheats the material to between 250xc2x0 C. and 750xc2x0 C.
In such a process one exploits the ability to heat the material by working it so that it passes the recrystallization temperature. Thus the material goes during the cold working through two phases. First, that is on moving from near-ambient temperature as the cold-working starts, the material reaches a high temperature which rigidifies the material. With the subsequent deformation and the further temperature increase the workpiece reaches its highest temperature, normally in the neighborhood of 700xc2x0 C. to 800xc2x0 C. At this temperature there is a sudden recrystallization of the material, typically crated by a thickness reduction of about 70%. The resultant crystalline structure is like that produced by annealing and results in a relatively soft material, that is one with limited rigidity and resistance to deformation.
Recrystallization is often desirable since it makes subsequent working easier. For example if the tubing is to be formed with ribs, it is much easier to do this when the metal is relatively soft and deformable, as happens when internal stresses are relieved by heating above the recrystallization temperature.
In other applications the recrystallized grain structure is not wanted, for example when producing tubing for use in plumbing installations. A tubular 85/15 (i.e. 85 mm diameter and 15 mm wall thickness) SF copper workpiece is Pilger rolled out to a 58/2.4 tube and then drawn or stretched to 15/1. Pilger rolling entails the use of a reciprocal broach or mandrel inside the tubular workpiece and continuously orbiting rolls on the outside, with step-wise advance of the tube coordinated with longitudinal shifting of the broach. In some systems the work-piece can even be drawn to a 6.35/0.3 finished product. The cold Pilger-rolled tube is not heated in this process.
Copper is a workpiece which can be drawn out considerably without heating. In the above-given example to produce a 6.35/0.3 tube the overall stretching or reduction is equal to 548:1. Thus Pilger rolling is the standard system for making copper tubing. In the Pilger system the tubing is coated with a cooling emulsion so the tube leaves the rolls at a temperature less than 100xc2x0 C. A disadvantage of this system is that it is impossible to feed the tubing from the planetary rolls directly to a drum-type drawing machine.
Hence the freshly rolled tubing is deposited horizontally in baskets that are carried on a conveyor system and that can raise or be raised as required to deliver the tubing at the desired level. Thus the baskets are not fixed on the conveyor but just set or hung thereon. Soft freshly rolled tubing is often damaged or marred by contact with the holding basket and other tubes when being transported to the subsequent rolling equipment. The amount of damage increases as the quantity being transported increases and is particularly high in drawing machines that must be fed a continuous supply of tubes at high speed.
Thin-walled air-conditioning/refrigeration (ACR) tubes having a wall thickness of about 0.3 mm is particularly susceptible to damage; it must be produced from a perfectly smooth workpiece. Such tubing is processed vertical in an angled planetary-roll system so that it can be fed to the slower vertical-throughput drawing machine used to make this type of tubing. The hanging bundles are normally transported by hook-type chain conveyors that are installed fixedly between the rolling and drawing installation. Thus a single-use transport system connects to a slow drawing installation. This is only true for soft tubes produced by normal angled planetary-roll systems. Were the tubes harder, it would be possible to use other transport systems that are less gentle, since the tubing would be less susceptible to damage. For instance if ACR tubing were being made from harder tubing, it could be transported from the planetary-rolling system to the drawing equipment in standard basket-type conveyors.
With Pilger rolling of cold stock the workpiece is advanced in steps. In order to produce dimensionally accurate tubing, the tubular workpiece is advanced in steps and rotated with each step through an angle of for example 57xc2x0. This advancing and rotating is effected outside the rolling system and can only be done in the brief instant when the tube is released by the Pilger rolls. The available time is very short so that the workpiece must be moved vary rapidly. Hence the mass of the workpiece is limited. In standard practice the maximum workpiece weight is therefore about 550 kg. This results in a production bottleneck, since at the Pilger-rolling equipment the workpiece must be reduced to a relatively small part and slowed down.
German patent document 1,752,996 of Dmitrijew describes a Pilger-rolling system wherein the rolls are reciprocated by a rack and thus rotated forward and backward. In this system the rolls slip on the workpiece since the drive gear radius is constant and the effective roll radius changes. The result is a poor finish on the tubing. Thin-walled tubing cannot be subjected to the longitudinal tension applied in such a system. Furthermore the discontinuous production results in considerable wear of all the machine parts, increasing down time and manufacturing costs.
In general, discontinuous production, that is movement of the workpiece in steps, creates numerous production problems, in particular when thin-walled ACR tubing is being made. The use of angled planetary rolls has the considerable advantage that it is usable in a continuous process with the workpiece never actually stopping. Thus there is no upper limit on overall workpiece size and in fact the workpiece can be rolled out as it issues from a continuous-casting die. Workpieces weighing in excess of 750 kg can easily be rolled and drawn.
Such a continuous process has, however, the considerable disadvantage that the extreme size reduction in a single stage heats the workpiece to 700xc2x0 C. to 800xc2x0 C., well above the recrystallization temperature. As a result the rolled workpiece is soft and weak so that it is easily damaged when subsequently handled and can only be subject to limited tension in a drawing operation.
It is therefore an object of the present invention to provide an improved system for making nonferrous tubing.
Another object is the provision of such an improved system for making nonferrous tubing which overcomes the above-given disadvantages, that is which continuously produces high-strength nonferrous tubing of relatively small wall thickness.
Seamless tubing of a nonferrous material is made by continuously and concomitantly advancing a tubular workpiece of the nonferrous material along an axis through a rolling station, radially squeezing the workpiece in the station between external rolls and an internal mandrel to radially reduce a wall thickness of the workpiece and increase an axial length of the workpiece so that the workpiece is heated, and spraying a liquid coolant against the workpiece in and upstream of the station to maintain the workpiece at a temperature below a recrystallization temperature of the material.
Thus the intermediate product produced by this system is quite hard and rigid. It is less likely to be damaged by handling, and can be subjected to considerable tension to draw it out and make its wall thickness even thinner. The production system works continuously, that is with the workpiece moving continuously at a constant speed through the rolling station so that the production speed is relatively high. The system according to the invention therefore has the advantages of the discontinuous Pilger system and those of the continuous angled planetary-roll system.
According to the invention the liquid coolant is sprayed annularly against the workpiece at a rate so as to carry off heat from the workpiece at a rate equal to more than 10,000 W/m2K. The rolls engage the workpiece over an axially extending zone having a predetermined length and the liquid is sprayed against the workpiece over an axially extending zone having a length equal to at least twice the predetermined length.
Furthermore in accordance with the invention the workpiece is internally sprayed with another liquid coolant upstream of the rolls. This other liquid coolant is pure water and is sprayed at a rate such that it is fully vaporized, leaving no liquid in the workpiece.
The workpiece according to the invention is advanced at a speed of V m/sec. The rolls engage the workpiece in a zone having an axial length of U m and with a force axial lengthening the workpiece by an amount L, and the workpiece spends a time T sec passing through the zone according to the formula T=2U/(V+V/L)+U/(V/L). The time T is greater than or equal to 2.5 sec.
In accordance with another feature of the invention the workpiece is contacted with a third liquid coolant downstream of the rolls to cool the workpiece to at most 100xc2x0 C. This third liquid coolant contacts the workpiece immediately downstream of the rolls. The workpiece is confined according to the invention at the rolls in an atmosphere of an inert gas to prevent oxidation of the metal. The third liquid coolant contacts the work-piece where it is confined in the inert gas.
The workpiece in accordance with the invention has upstream of the rolls a ratio of diameter to wall thicknessxe2x89xa75:1. The rolls axially lengthen the workpiece to an extentxe2x89xa68.